"You'll need a new app to open this file" is probably the most common error faced by millions of Windows users every day. This is either because the program required to open a file is not available on your system or there may be other issues.

What is .HMK file extension?

Greeting card project created with Hallmark Card Studio; may be a birthday card, invitation, letter, or other kind of card; can include images (user photos or pictures bundled with the program) and custom-formatted text.
For a professional look, cards created with Hallmark Card Studio can be printed on Hallmark Premium Blank Greeting Cards paper, a 85 lb heavyweight card stock with a coated matte finish.
You can see a list of software programs here on easyfileopener.org that can open this file type so that you don’t need to search for it on the web. You can then open your file with this extension using a software listed below.